1. Field of the Invention
Priority is claimed on Japanese Patent Application No. 2004-349689, filed Dec. 2, 2004, the content of which is incorporated herein by reference.
The present invention relates to a piston for an internal combustion engine, more specifically to a technique for forming a resinous coating layer on a skirt part surface so as to reduce a frictional resistance of the piston.
2. Description of Related Art
It has been known that surface treatment of a sliding surface of a piston of an internal combustion engine improves fuel efficiency and prevents seizure and abnormal noise generation. For example, a surface treatment has been proposed in which minute concaves are provided on a skirt part outer wall by sand blasting or shot peening (e.g. cf. Patent Reference 1). Patent Reference 1 proposes that lubrication oil is retained in those minute concaves to hardly flow down such that seizure of the cylinder liner and piston can be prevented.
As a similar surface treatment to that of Patent Reference 1, a piston for a 2-stroke cycle gasoline engine with a number of concaves molded by pressurized molding processing, sand blasting, shot peening, and so on, in an outer circumferential surface of the skirt part (e.g., cf. Patent Reference 2), has been proposed. Patent Reference 2 proposes that since, by providing concaves on the skirt part of the piston, sufficient lubricating oil is supplied to the upper portion of the cylinder inner wall surface exposed to the high temperature gas, lubrication performance is improved over the entire sliding surface between the piston and cylinder.
There has been also disclosed a piston in which a protruding part extending along the circumferential direction is provided on the skirt part of the side surface of the piston body and further minute dimples (concaves) are provided on the surface of this protruding part (e.g. cf. Patent Reference 3). These minute dimples are formed in the appropriate size by shot peening. According to Patent Reference 3, the lubricating oil is not only retained in respective minute dimples but also is easily supplied from coves part formed on concaves to the sliding surface. Accordingly, after a primary abrasion is finished and the piston adapts itself to the sleeve, the lubricating oil is supplied to the sliding surface to form an oil film so as to enable not only the lubrication of the sliding surface but also prevention of abnormal noise generation.
Further, there has been disclosed a sliding part element on which a coating layer is formed after streaks are formed on the surface of basic material for the sliding part element such that the surface roughness thereof becomes 8 μm Rz to 18 μm Rz by the 10 point average roughness measuring (e.g. cf. Patent Reference 4). Specifically, the coating layer is composed of a dry coating film lubricant containing at least one type of coating film modifier selected from polyamide-imide resin, epoxy silane and epoxy resin and at least one type of hard particle selected from silicone nitride and alumina. According to Patent Reference 4, since an abrasion resistance and close contact of the sliding part element are improved while a friction coefficient of the sliding part element is reduced by the aforementioned coating layer, an invention of Patent Reference 4 can be applied to pistons.
[Patent Reference 1] Japanese Utility Model Patent Laid-Open Publication No. S 52-16451.
[Patent Reference 2] Japanese Utility Model Patent Laid-Open Publication No. S 57-193941.
[Patent Reference 3] International Patent Laid-Open Pamphlet No. 01/002717.
[Patent Reference 4] Japanese Patent Laid-Open Publication No. 2004-149622.
As described above, in Patent References 1 through 3, minute concaves are provided on the sliding surface of the skirt part made of metal by mechanical processing such as sand blasting, shot peening and pressurized molding to be used as an oil reservoir from which oil is lubricated to the sliding surface so as to reduce the friction of the skirt part. However, in the sliding region of the skirt part in a poor oil lubrication state, what is called the solid contacting region, the effect of reduction of the friction is not achieved by the lubrication oil. Therefore, the surface treatment according to Patent References 1 through 3 was limited to reduction of the frictional resistance at the skirt part.
In Patent Reference 4, a frictional resistance in the solid contact region is reduced by coating the skirt part with a dry coating film lubricant. On the sliding surface of the skirt part, regularly arranged streaks are formed by cutting processing before the coating and serve as the oil reservoir to expand the oil-circulating region, what is called the complex lubrication region in the skirt part so as to reduce the frictional resistance. However, Patent Reference 4, when forming the coating layer, finishes the sliding surface of the basic material to a desired surface roughness in advance by the usual mechanical processes such as grinding and lathing, further forms regularly arranged streaks by cutting process, subsequently applies the coating layer, cures it by the reaction at high temperature so as to form the coating layer with the dry coating film lubricant. That is, although Patent Reference 4, compared to Patent References 1 to 3, is expected to be more effective in reducing the friction resistance of the skirt part, it includes two processes, mechanical and chemical (coating) surface treatments, in the surface treatment of the skirt part. Therefore, there has been a strong demand for a surface treatment method enabling the reduction of frictional resistance of the skirt part more easily.